In the central nervous system (CNS) of higher vertebrates, regeneration of axons after injury is almost absent and structural plasticity is limited. Growth inhibitors associated with CNS myelin are likely to play an important role. This is evidenced by a monoclonal antibody (mAb), IN-1, that neutralizes a potent neurite growth inhibitory myelin protein, thereby promoting long-distance axonal regeneration and enhancing compensatory plasticity following spinal cord or brain lesions in adult rats.
A number of in vitro and in vivo observations have revealed a new aspect of neurite growth regulation which is the presence of repulsive and inhibitory signals and factors (Keynes and Cook, 1995, Curr. Opin. Neurosci. 5:75-82). Most of these signals seemed to be proteins or glycoproteins. A first breakthrough towards identification of the factors was the purification and cDNA cloning of a chick brain-derived growth cone collapse inducing molecule, Collapsin-1, now called Semaphorin 3A.
A second group of repulsive guidance cues recently purified and cloned is now designated as Ephrins. They are ligands for the Eph receptor tyrosine kinase family. Ephrin-A5 and Ephrin-A2 are expressed as gradients in the optic tectum of the chick embryo, and their ectopic expression or deletion causes guidance errors of ingrowing retinal axons. Like the Semaphorins, the Ephrin family has 15 to 20 members, each with a complex and dynamic expression in and outside of the nervous system. The functions of most of these molecules remain to be analyzed.
A third group of guidance cues which can repulse growing axons and is expressed in the developing nervous system are the Netrins. Netrin has been purified as a floor plate derived chemoattractant for commissural axons in early spinal cords, like its C. elegans ortholog unc-6. Netrin-1 turned out to have repulsive effects for certain types of neurons depending on the type of receptor present on the target neuronal growth cones (Tessier-Lavigne et al., 1996, Science 274:1123-33).
Previously, a potent neurite growth inhibitory activity associated with adult CNS oligodendrocytes and myelin was reported by Canoni and Schwab (1988, J. Cell Biol. 106:1281-1288). A major constituent is a high molecular weight membrane protein (NI-250, with a smaller component, NI-35, in rat) which was recently purified, and which is related to the subject of the present invention, and is bound by the neutralizing mAb, IN-1 (Canoni and Schwab, 1988, J. Cell Biol. 106:1281-1288; U.S. Pat. Nos. 5,684,133; 5,250,414; PCT Publication WO 93/00427).
Myelin-associated neurite growth inhibitors play a crucial role in preventing regeneration of lesioned CNS axons. When oligodendrocyte development and myelin formation is blocked in chicken or rats, the regeneration permissive period following CNS lesions is prolonged. Indeed, myelin formation coincides in time with the end of the developmental period where the CNS shows high structural plasticity and a high potential for regeneration.
NI-250 and NI-35 are likely to be major components of the myelin-associated growth inhibition as evidenced by in vivo application of IN-1 to spinal cord lesioned adult rats which induced regeneration of corticospinal axons over long distances and allowed motor and behavior functional recovery especially with regard to locomotion. Similar experiments on the optic nerve and the cholinergic septo-hippocampal pathway also demonstrated the in vivo relevance of the IN-1 recognized antigen, NI-35/250 (Schnell and Schwab, 1990, Nature 343:269-272; Bregman et al., 1995, Nature 378:498-501).
Unlesioned fiber systems also respond to the neutralization of neurite growth inhibitors by IN-1. Recent experiments have conclusively shown that following a selective corticospinal tract lesion (pyramidotomy), intact fibers sprout across the midline in the spinal cord and brainstem and establish a bilateral innervation pattern, accompanied by an almost full behavioral recovery of precision movements in the presence of IN-1 (Z'Graggen et al., 1998, J. Neuroscience 18(12):4744-4757).
Isolation of the gene that encodes the neurite growth inhibitory protein provides multiple opportunities for developing products useful in neuronal regeneration and in treatment of various neurological disorders, including CNS tumor.
Citation of a reference hereinabove shall not be construed as an admission that such reference is available as prior art to the present invention.